I would also recommend registering on a website called
www.airquiz.com
They have a huge database of genuine questions that they use to generate mock theory exams. For a one off £20 you get unlimited attempts for 2 years.
The papers are marked on line and the ones you got wrong are sent to you by email, sometimes with links to CAA or AIS websites.
Like you I enjoy the flying, but find studying again at my age (41) very hard to discipline myself. So I decided to take 3 months off flying during winter to do the theory. I've passed Met and RT Comms theory, doing Human Perfomance next weekend, and then Navigation at the end of Feb before I start flying again - just in time for my first solo nav. Then it'll just be Aircraft General & Flight Planning to do.
I already feel a massive load off my shoulders as I plough through the books while its raining outside!
